Method for forming a thin film

ABSTRACT

A reactive solution with an amount of 250 mL is made of distilled water and LiOH·H 2 O (4M) melted in the distilled water. Then, the reactive solution is put in a flow-type reactor, and is flown in between an anode electrode and a cathode electrode set in the flow-type reactor at a given temperature and a given flow rate. Then, a given voltage is applied between the anode electrode and the cathode electrode with dropping an oxidizer of hydrogen peroxide (H 2 O 2 ) into the reactive solution to form a lithium-cobalt oxide thin film on the anode electrode.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method for forming a thin film, particularlyto a method for forming a thin film suitable for a secondary batteryfield usable for mobile electronic device and electric automobiles.

2. Related Art Statement

So far, a sol-gel method, a CVD method or a PVD method is employed as athin film-forming method. These methods require multistage processincluding a heating process, a high vacuum condition, or a high energycondition accompanied with a substrate-heating process or aplasma-generating process. Therefore, those methods require large scaleand complicate apparatus, resulting in large cost and complicateoperationality in use.

Moreover, the above high energy condition runs counter to globalenvironmental protection, resource saving and energy saving. Therefore,a new thin film-forming method without the above high energy conditionhas been desired.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a new thinfilm-forming method not including a high energy condition due to firing,heating or plasma generation.

This invention relates to a method for forming a thin film (hereinafter,often called as a “first thin film-forming method) comprising the stepsof:

setting an anode electrode and a cathode electrode in a reactivesolution, flowing the reactive solution in between the anode electrodeand the cathode electrode at a given flow rate, and

applying a given voltage between the anode electrode and the cathodeelectrode, thereby to synthesize a compound thin film including thecomponents of the reactive solution and the anode electrode on the anodeelectrode.

This invention also relates to a method for forming a thin film(hereinafter, often called as a “second thin film-forming method)comprising the steps of:

setting an anode electrode and a cathode electrode in a reactivesolution,

flowing the reactive solution in between the anode electrode and thecathode electrode at a given flow rate, and

applying a given voltage between the anode electrode and the cathodeelectrode, thereby to synthesize a compound thin film including thecomponents of the reactive solution and the anode electrode on a givensubstrate arranged in the flow direction of the reactive solution.

The inventors related to the present invention have been intenselystudied for developing a new thin film-forming method not including ahigh energy process. As a result, they have found out surprisingly thatwhen a reactive solution incorporating a component of a desired thinfilm is flown in between an anode electrode and a cathode electrode towhich a given voltage is applied, the desired compound thin filmincluding the component of the reactive solution and another componentof the anode electrode is synthesized directly on the anode electrode.Then, they have also found out that when the anode electrode and thecathode electrode are set in a static reactive solution and a givenvoltage is applied between the anode electrode and the cathodeelectrode, the compound thin film is not synthesized on the anodeelectrode.

The first thin film-forming method of the present invention is made onthe discovery of the above phenomenon. The first thin film-formingmethod enables the desired compound thin film to be formed directly onthe anode electrode without the high energy condition including thesubstrate-heating process and the plasma generation.

Moreover, the inventors have found out that if the flow rate of thereactive solution and the voltage applied between the anode electrodeand the cathode electrode are controlled appropriately, the compoundthin film composed of the components of the reactive solution and theanode electrode can be synthesized on a given substrate which isarranged in the flow direction of the reactive solution.

The cause of the thin film formation on the given substrate may beconsidered as follows: That is, the reactive solution erodes and meltsthe superficial parts of the anode electrode, and thus, the melted anodeelectrode material and the reactive solution arrive at and synthesizedon the substrate.

The second thin film-forming method of the present invention is made onthe discovery of the above phenomenon. The second thin film-formingmethod enables the compound thin film to be formed on the givensubstrate without the high energy condition including thesubstrate-heating process and the plasma generation.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more particularly described with reference to theaccompanying drawings:

FIG. 1 is a graph showing a X-ray diffraction spectrum of alithium—cobalt oxide thin film synthesized by the thin film-formingmethod of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention will be described in detail as follows.

In the first thin film-forming method of the present invention, the flowrate of the reactive solution is not restricted even if the compoundthin film, composed of the components of the reactive solution and theanode electrode, can be synthesized on the anode electrode. The flowrate of the reactive solution is determined on the kind of the reactivesolution, the anode electrode material, the kind of the compound thinfilm to be formed and the forming speed of the compound thin film.

However, it is preferable that the upper limit value of the flow rate ofthe reactive solution is 100 mL/minute, particularly 20 mL/minute.Moreover, it is preferable that the lower limit value of the flow rateof the reactive solution is 1 mL/minute, particularly 5 mL/minute.Thereby, irrespective of the kinds of the reactive solution and theanode electrode material or the like, the compound thin film can beformed on the anode electrode uniformly.

The voltage to be applied between the anode electrode and the cathodeelectrode is not restricted even if the compound thin film is formed onthe anode electrode according to the present invention. However, it ispreferable that the voltage is applied so that a current with a range of0.01-5 mA/cm³ is flown between the anode electrode and the cathodeelectrode. Thereby, the compound thin film can be formed uniformly withthe enhancement of the thin film-forming speed.

In the second thin film-forming method of the present invention, theflow rate of the reactive solution is not limited even if the compoundthin film, composed of the components of the reactive solution and theanode electrode, can be formed on the given substrate.

However, it is desired that the upper limit value of the flow rate ofthe reactive solution is 200 mL/minute, particularly 50 mL/minute.Moreover, it is desired that the lower limit value of the flow rate ofthe reactive solution is 2 mL/minute, particularly 5 mL/minute. Thereby,the reactive solution can erode the superficial parts of the anodeelectrode effectively, and transport the thus melted anode electrodematerial on the given substrate.

As mentioned in the first thin film-forming method, it is desired thatthe voltage is applied between the anode electrode and the cathodeelectrode so that a current with a range of 1-5 mA/cm³ is flowntherebetween. The reactive solution is selected on the kind of thecompound thin film to be formed.

In the second thin film-forming method of the present invention, thecathode electrode and the anode electrode may be made of a well knownelectrode material. Moreover, the substrate may be made of any materialin any shape, depending on the use of the compound thin film.

Moreover, in the first thin film-forming method of the presentinvention, the cathode electrode may be made of a well known electrodematerial, and the anode electrode may be made of a given materialdepending on the kind of the compound thin film. For example, information of a lithium—cobalt thin film, a cobalt electrode may be usedas the anode electrode.

If an oxide thin film is formed on the anode electrode according to thethin film-forming method of the present invention, it is desired toincorporate an oxidizer in the reactive solution. Thereby, thecomponents of the reactive solution and the anode electrode can beeasily oxidized, the oxide thin film can be formed on the anodeelectrode or the substrate uniformly in a short time.

Hydrogen peroxide (H₂O₂) and Na₂S₂O₃ may be exemplified as the oxidizer.Particularly, in the formation of the lithium-cobalt thin film, thehydrogen peroxide is preferably used.

EXAMPLES

This invention will be concretely described with reference to thefollowing examples.

First of all, a cobalt electrode as the anode electrode and a platinumelectrode as the cathode electrode were set in a flow-type reactor.

Then, a reactive solution with an amount of 250 mL was made of distilledwater and LiOH·H₂O (4M) melted in the distilled water. The reactivesolution was put in the flow-type reactor, and then, held at 150° C. andflown in between the cobalt electrode and the platinum electrode at aflow rate of 5 mL/minute.

Then, a given voltage was applied between the electrodes so that acurrent density of 1 mA/cm³ was flown therebetween, and hydrogenperoxide (H₂O₂) was dropped off into the reactive solution at a rate of1 mL/minute.

Three hours later, the cobalt electrode was taken out of the flow-typereactor, and the surface of the cobalt electrode was analyzed by X-raydiffraction. The thus obtained X-ray diffraction pattern was shown inFIG. 1.

As is apparent from FIG. 1, diffraction peaks from hexagonal LiCoO₂(R3m) are observed on the surface of the cobalt electrode. That is, itis turned out that a compound thin film made of LiCoO₂ crystal is formedon the cobalt electrode as the anode electrode.

Moreover, it is turned out that from SEM observation, the compound thinfilm has a layered structure.

Although this invention has been described in detail with reference tothe above examples, this invention is not limited to the abovedisclosure and every kind of variation and modification can be madewithout departing from the scope of the present invention.

According to the present invention, a compound thin film can be directlysynthesized on an anode electrode or a given substrate at roomtemperature. Therefore, a high energy condition including a hightemperature substrate heating process, a plasma generation or the likeis not required, different from a conventional CVD method and PVDmethod. As a result, a new thin film-forming method not including thehigh energy condition can be provided.

What is claimed is:
 1. A method for forming a thin film comprising thesteps of: setting an anode electrode, cathode electrode, and a substratein a reactive solution, flowing the reactive solution in between theanode electrode and the cathode electrode at a given flow rate, andapplying a given voltage between the anode electrode and the cathodeelectrode, thereby to synthesize a compound thin film including thecomponents of the reactive solution and the anode electrode on the givensubstrate arranged in the flow direction of the reactive solution.
 2. Amethod for forming a thin film as defined in claim 1, wherein the flowrate of the reactive solution is set within 2-200 mL/minute.
 3. A methodfor forming a thin film as defined in claim 1 or 2, further comprisingthe step of incorporating an oxidizer in the reactive solution, wherebyan oxide thin film including the components of the reactive solution andanode electrode is formed on the given substrate.
 4. A method forforming a thin film comprising the steps of: setting an anode electrodecomposed of a cobalt electrode and a cathode electrode in a reactivesolution composed of a lithium hydroxide water solution, incorporatingan oxidizer in the reactive solution, flowing the reactive solution inbetween the anode electrode and the cathode electrode at a given flowrate, and applying a given voltage between the anode electrode and thecathode electrode, thereby to synthesize a lithium cobalt thin film asan oxide thin film including the components of the reactive solution andthe anode electrode on a given substrate arranged in the flow directionof the reactive solution.
 5. A method for forming a thin film as definedin claim 4, wherein the oxidizer is hydrogen peroxide.
 6. A method forforming a thin film as defined in claim 4, wherein the flow rate of thereactive solution is set within 2-200 mL/minute.